1. Field of the Invention
The invention relates to a process and apparatus for cooling a fast fluidized bed catalyst regenerator used for regeneration of fluidized catalytic cracking catalyst.
2. Description of Related Art
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425C-600C, usually 460C-560C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper, and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500C-900C, usually 600C-750C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 40's. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is reported in Fluid Catalytic Cracking Report by Amos A. Avidan, Michael Edwards and Hartley Owen, as reported in the Jan. 8, 1990, edition of the Oil & Gas Journal.
Modern catalytic cracking units use active zeolite catalyst to crack the heavy hydrocarbon feed to lighter, more valuable products. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, much less contact time is needed. The desired conversion of feed can now be achieved in much less time, and more selectively, in a dilute phase, riser reactor.
Although reactor residence time has continued to decrease, the height of the reactors has not. The need for a somewhat vertical design to accommodate the great height of the riser reactor and the need to have a unit which is compact, efficient, and has a small "footprint" has caused considerable evolution in the design of FCC units, which evolution is reported to a limited extent in the Jan. 8, 1990, Oil & Gas Journal article. One modern, compact FCC design is the Kellogg Ultra Orthoflow converter, Model F, which is shown in FIG. 1 of this patent application, and also shown as FIG. 17 of the Jan. 8, 1990, Oil & Gas Journal article discussed above. The unit uses a regenerator consisting of a bubbling dense bed of catalyst. The regenerator has an external heat exchanger, which allows the unit to process heavy crudes or those containing large amounts of Conradson Carbon Residue material without overheating.
Such cooling of the catalyst regenerator is ancient art. In the 40's many FCC regenerators had external catalyst coolers. With better unit design and more active catalyst the units ran "heat balanced" and did not require catalyst coolers. Today FCC units are being pushed to crack heavier and heavier feeds, which contain large amounts of CCR, so once again catalyst coolers are needed to permit heat balanced operation.
Coolers are now usually connected to the bubbling dense bed of catalyst in the regenerator, as in the Kellogg HOC design. These bubbling dense beds regenerate catalyst and store it until recycled to the riser reactor. Not all regenerators use bubbling dense bed as the primary means of removing carbon from spent catalyst.
The "High Efficiency Regenerator" (H.E.R.) design uses a fast fluidized bed for most of the coke combustion and a dilute phase transport riser for some CO combustion. Regenerated catalyst is collected in a bubbling dense bed for reuse and for recycle to the coke combustor. Such a design makes more efficient use of the catalyst in that the coke combustor is all highly active, unlike bubbling dense bed regenerators, which are troubled with stagnant beds (due to poor catalyst flow patterns) and regeneration gas bypassing (due to the formation of large bubbles within the bubbling dense bed). The H.E.R. design uses a turbulent fluidized bed or a fast fluidized bed, which allows use of much less catalyst inventory than is required in a bubbling dense bed regeneration design.
For maximum effectiveness the H.E.R. design requires recycle of hot regenerated catalyst from the bubbling dense bed to the coke combustor of fast fluidized bed (FFB) region to heat it. Heating of the FFB region is beneficial, and usually essential, to rapidly heat up the spent catalyst to a high enough temperature so that coke combustion proceeds rapidly. Recycle of regenerated catalyst also creates something of a flywheel effect, and provides a constant source of high temperature catalyst to heat the incoming relatively cooler spent catalyst.
H.E.R. units have, like bubbling bed units, been pushed to deal with the increased carbon burning duties associated with cracking of heavy crudes. The operation of these units has been modified to try to maintain heat balanced operation, either by limiting the heat release during regeneration (partial CO combustion mode) or by removing heat via heat exchange.
A partial combustion route to limiting heat release is disclosed in U.S. Pat. No. 4,849,091, which is incorporated herein by reference. Such an approach allows some of the heat release to be shifted to a downstream CO boiler.
Adding heat exchangers to H.E.R. regenerators in various ways has been reported in the patent literature. Most of the heat exchangers are connected to the bubbling dense bed associated with the H.E.R. regenerator.
In U.S. Pat. No. 4,439,533 a backmixed heat exchanger is added to the bubbling dense bed of catalyst. There are no slide valves or elaborate catalyst supply and return lines to the heat exchanger, rather the heat exchanger is closely coupled, and in open fluid communication with the bubbling dense bed. The backmixed heat exchanger looks something like a thimble, and for convenience such a backmixed, close coupled heat exchanger which is in open fluid communication with a catalyst regeneration zone may be hereafter referred to as a "thimble" heat exchanger. Adjustment of the amount of fluidizing gas added to the "thimble" containing the heat exchange tubes allows control of catalyst circulating in the thimble and of heat exchange. Cooling fluid, usually water, is passed through a tube bundle within the thimble. Cooled catalyst is returned to the bubbling dense bed regenerator.
In U.S. Pat. No. 4,434,245 a flow-through heat exchanger is supplied having a hot catalyst inlet in the bubbling dense bed, and a cooled catalyst outlet in the coke combustor or FFB region. This approach requires a significant amount of hardware modifications, slide valves, and a fluidizing air outlet from the upper portion of the heat exchanger to the dilute phase region above the bubbling dense bed.
In U.S. Pat. No. 4,578,366 a flow-through heat exchanger is used, with fluidizing gas used in the heat exchanger also being used to support combustion in the coke combustor. Catalyst slide valves are shown for regulating the flow of hot regenerated catalyst from the bubbling dense bed into the heat exchanger.
In U.S. Pat. No. 4,595,567 a flow-through heat exchanger is used, with heat pipes in the heat exchanger. Catalyst slide valves regulate the flow of hot regenerated catalyst from the bubbling dense bed into the heat exchanger.
Only one reference is known with a heat exchange means within, or connected to, the fast fluidized bed region of an H.E.R. In U.S. Pat. No. 4,430,302 a fast fluidized bed regenerator (without catalyst recycle) is shown with looped heat exchange coils within the fast fluidized bed. Looped coils formed from 11/2 or 2" 304H stainless steel were suspended in multiple banks within a reaction vessel having a substantially vertical sidewall. The length of horizontal sections of heat exchange tube was minimized to minimize erosion of the tubes.
We reviewed this extensive art on putting coolers around FCC regenerators but found nothing that was completely satisfactory.
We wanted to avoid use of slide valves to control catalyst flow to the regenerator. These can cost more than $1,000,000 each and are usually used in pairs to permit servicing.
We wanted to avoid coils within the FFB region of the regenerator. Cooling coils must always be full of coolant (to avoid thermal shock and damage to the tubes). Coils full of coolant will remove heat even during startup, when the unit requires heating, not cooling. There is concern too that coils may interfere with fluidization within the FFB region.
We needed a reliable and efficient way of removing heat from an FFB regenerator during normal operation, which could be safely isolated from the FFB region during startup. We wanted to make double use of any gas added to the heat exchanger to promote fluidization, i.e., use it once to control the heat transfer coefficient in the heat exchanger, and use it again to supply combustion air.
We wanted a way to limit as much as possible the temperature rise experienced by spent catalyst particles in FFB regenerators. We wanted more than a heat sink which would absorb heat from regenerating catalyst in the riser above the FFB. We needed a way to cool at least some of the catalyst during the regeneration process.
We also wanted to have the option to remove some heat from the coke combustor, without "linkage" to the amount of catalyst recycle. Although catalyst recycle to the coke combustor is usually essential to "fire up" the coke combustor, it causes some problems. Catalyst recycle from the bubbling dense bed to the FFB increases the solids traffic in the dilute phase region above the bubbling dense bed, and this can cause increased emissions of particulates. Large amounts of catalyst recycle require work in raising and lowering of tons of catalyst from the FFB region to the second dense bed. Catalyst recycle also dilutes the carbon on catalyst content in the coke combustor which, although beneficial in limiting temperature rise, makes it harder to achieve low levels of carbon on regenerated catalyst. Coke burns quickly when the apparent carbon concentration on catalyst is high, and slowly when carbon concentration is low.
We realized that putting a catalyst cooler beneath but in open fluid communication with the fast fluidized bed region we could obtain several benefits. The benefits can be summarized as higher heat transfer rates, multiple use of air, reduced temperature rise downstream of the coke combustor, and some cooling of partially regenerated catalyst during the regeneration process. We discovered that a backmixed cooler beneath the fast fluidized bed region can also give higher heat transfer rates than a backmixed cooler connected to a bubbling dense bed.